1. Field of the Invention
The present invention relates in general to audio amplifiers and in particular a method and apparatus for obtaining off-line direct audio from a power supply without the use of a relatively high-power amplifier circuit.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Audio circuits are well known in the prior art and generally utilize an audio amplifying circuit to drive the audio speaker. The audio amplifier is required to a be power audio stage. Such power amplifier circuits are illustrated in U.S. Pat. Nos. 4,409,559 and 4,651,112.
A typical class AB audio amplifier has a power supply and the audio amplifier. The cost of these two units together is significant in the manufacture of an audio output device. There are many switching power amplifiers that attempt to avoid the requirement of an audio power amplifier in the circuit.
Such circuits are shown in U.S. Pat. Nos. 4,763,080, 4,517,522, 4,047,120 and 4,453,131.
Class D amplifiers are always switching at fifty percent duty cycle even when no signal is applied. This causes an inherent noise that increases the signal-to-noise ratio when no signal is applied.
It would be advantageous to have a highly efficient effective audio output circuit deriving its energy from a power supply and avoiding the requirement of having an expensive audio amplifier as the final stage.